Photonic lock based high bandwidth photodetector

ABSTRACT

The technique introduced herein decouples the traditional relationship between bandwidth and responsivity, thereby providing a more flexible and wider photodetector design space. In certain embodiments of the technique introduced here, a photodetector device includes a first mirror, a second mirror, and a light absorption region positioned between the first and second reflective mirrors. For example, the first mirror can be a partial mirror, and the second mirror can be a high-reflectivity mirror. The light absorption region is positioned to absorb incident light that is passed through the first mirror and reflected between the first and second mirrors. The first mirror can be configured to exhibit a reflectivity that causes an amount of light energy that escapes from the first mirror, after the light is reflected back by the second mirror, to be zero or near zero.

RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 13/709,209, entitled “HIGH-EFFICIENCY BANDWIDTHPRODUCT GERMANIUM PHOTODETECTOR,” filed on Dec. 10, 2012, which claimspriority to Taiwan Patent Application No. 101144392, filed on Nov. 27,2012; all of which are incorporated by reference herein in theirentireties.

This application also claims the benefit of U.S. Provisional PatentApplication No. 61/921,412, entitled “PHOTONIC LOCK BASED HIGH BANDWIDTHPHOTODETECTOR,” filed on Dec. 28, 2013; and U.S. Provisional PatentApplication No. 61/929,112, entitled “PHOTONIC LOCK BASED HIGH BANDWIDTHPHOTODETECTOR WITH VARIOUS REFLECTOR DESIGNS,” filed on Jan. 19, 2014;both of which are incorporated by reference herein in their entireties.

TECHNICAL FIELD

Embodiments of the present disclosure relate to photodetector designs,and more particularly, to photonic lock based high bandwidthphotodetectors.

BACKGROUND

With the ever increasing demand for high speed communication, there hasbeen a significant increase in the use of optics in computing systems.Semiconductor photonics, which can be implemented in an integratedcircuit with known fabrication techniques, are increasingly deployed tomeet the growing use of optics. The resulting semiconductorphotodetectors have a small form factor, and their detection bandwidthscan be very high. These traits make semiconductor photodetectorssuitable for optical fiber based, high-speed telecommunication anddatacenter interconnect applications.

However, in conventional photodetector designs, there is an inherenttrade-off between responsivity and bandwidth. Traditionally, theresponsivity is directly proportional to the path length that the lighttravels in the light absorption material (e.g., germanium). Consideringa conventional normal incidence photodetector as an example, the thickerthe absorption layer is, the higher the responsivity is. Unfortunately,a thicker absorption layer almost always features the drawback of longercarrier transit time that reduces the bandwidth of photodetectors.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the present disclosure are illustrated by wayof example and not limitation in the figures of the accompanyingdrawings, in which like references indicate similar elements. Thesedrawings are not necessarily drawn to scale.

FIGS. 1A-1B illustrate cross-sectional views of conventionalphotodetector structures.

FIG. 2 illustrates a cross-sectional view showing relevant details ofimplementing a photonic lock based photodetector in accordance with someembodiments.

FIG. 3 illustrates a flow diagram showing an example of a design processthat may be implemented by an electronic design automation (EDA)software application for designing a photonic lock based photodetector.

FIG. 4 illustrates a cross-sectional view of an example implementationof a butt-coupling photodetector incorporating the technique introducedhere.

FIG. 5 illustrates a cross-sectional view of an example implementationof an evanescent-coupling photodetector incorporating the techniqueintroduced here.

FIG. 6 illustrates a cross-sectional view of an example implementationof a normal-incidence photodetector incorporating the techniqueintroduced here.

FIGS. 7A-7H illustrate various examples of mirror designs, which can beadapted in the photonic lock based photodetectors.

FIG. 8 illustrates a high-level block diagram showing an example ofcomputing system in which at least some portion of the design process ofFIG. 3 can be implemented or in which an embodiment of the photonic lockbased photodetectors introduced here can be implemented.

DETAILED DESCRIPTION

It is observed that the aforementioned trade-off between responsivityand bandwidth in conventional photodetector designs is undesirable. Oneconventional attempt to overcome this drawback is to place a mirrorstructure (or a reflective region) adjacent to the light absorptionregion to reflect the light back to the light absorption region. FIGS.1A-1B illustrate the cross-sectional views of three differentphotodetector structures all implementing this conventional approach.Note that the structures shown in FIGS. 1A-1B, as well as in otherfigures throughout this disclosure, are for illustration purposes only;consequently, certain well-known structures (e.g., substrate) may beomitted in one or more figures for simplicity. The three different typesof photodetectors include a butt-coupling photodetector 100, anormal-incidence photodetector 102, and an evanescent-couplingphotodetector 104.

As shown, incident light enters a light absorption region (or lightabsorption cavity) 110 of the photodetectors 100, 102, and 104 at oneend of the light absorption region 110; a mirror structure (or areflective region) 130 can be positioned at the other end of the lightabsorption region 110 to reflect the incident light back to the lightabsorption region 110. While this approach improves the responsivitybecause the incident light's travel path doubles, the improvement islimited. Also, adding the mirror 130 may cause well-knownback-reflection issues. It is further observed that one important reasonfor this bandwidth and responsivity trade-off in the conventionalphotodetector designs is caused by the light not being confined insidethe light absorption region 110, and henceforth the absorption of lightcan only take place during a limited amount of light passes (which istwo in the above examples of FIGS. 1A-1B) inside the light absorptionregion 110.

Introduced here is a technique to overcome such trade-off by providing away to confine the incident light in the light absorption region 110 forreaching high responsivity without sacrificing bandwidth. Morespecifically, by confining the incident light, the total volume (e.g.,thickness) of the light absorption material can be greatly reducedwithout sacrificing much or even any responsivity, because the lightremains within the light absorption region 110 (or light absorptioncavity such as cavity layer 112 of FIG. 1B) until the light is absorbedby the light absorption material. This smaller size can bring thebenefit of higher bandwidth.

As described further below, the apparatus and technique introducedherein decouple the traditional relationship between bandwidth andresponsivity, thereby providing a more flexible and wider photodetectordesign space for next generation high-speed photodetector applications.In certain embodiments, a photodetector device includes a first mirror,a second mirror, and a light absorption region positioned between thefirst and second reflective mirrors. For example, the first mirror canbe a partial mirror, and the second mirror can be a high-reflectivitymirror. The incident light can be guided from a waveguide, through thefirst mirror, and then enter the light absorption region. The lightabsorption region is positioned to absorb incident light that is passedthrough the first mirror and reflected between the first and secondmirrors. The first mirror can be configured to exhibit a reflectivitythat causes an amount of energy of light escaped from the first mirror,after being reflected back by the second mirror, to be zero or nearzero. That is to say, in ways discussed herein, the present techniquecan configure the optical parameters of the first mirror together withthe second mirror and the light absorption region such that asubstantial amount of energy of the incident light can be captured by orconfined within the light absorption region. In this way, embodiments ofthe present technique can enjoy both high bandwidth and highresponsivity. Some photodetector embodiments of the present techniquemay achieve a bandwidth of over 40 Gbit/s as compared to 10-25 Gbit/s oftraditional photodetectors. Also, the thickness (e.g., in anormal-incidence photodetector) or the length (e.g., in a butt-couplingphotodetector) of the light absorption region may be reduced from 3 μmor thicker to around 200 nm.

Additionally, methods for designing a photonic lock based photodetector,which may be embodied by an electronic design automation (EDA) softwareapplication, are discussed below.

In the following description, numerous specific details are set forth toprovide a thorough understanding of the present disclosure. It will beapparent to one skilled in the art that the techniques introduced heremay be practiced without these specific details. In other instances,well-known features, such as specific fabrication techniques, are notdescribed in detail in order to not unnecessarily obscure the presentdisclosure. References in this description to “an embodiment,” “oneembodiment,” or the like, mean that a particular feature, structure,material, or characteristic being described is included in at least oneembodiment of the present disclosure. Thus, the appearances of suchphrases in this specification do not necessarily all refer to the sameembodiment. On the other hand, such references are not necessarilymutually exclusive either. Furthermore, the particular features,structures, materials, or characteristics may be combined in anysuitable manner in one or more embodiments. Also, it is to be understoodthat the various exemplary embodiments shown in the figures are merelyillustrative representations and are not necessarily drawn to scale.

The terms “coupled” and “connected,” along with their derivatives, maybe used herein to describe structural relationships between components.It should be understood that these terms are not intended as synonymsfor each other. Rather, in particular embodiments, “connected” may beused to indicate that two or more elements are in direct physical orelectrical contact with each other. “Coupled” may be used to indicatethat two or more elements are in either direct or indirect (with otherintervening elements between them) physical or electrical contact witheach other, and/or that the two or more elements co-operate or interactwith each other (e.g., as in a cause and effect relationship).

The terms “over,” “under,” “between,” and “on” as used herein refer to arelative position of one material layer with respect to other materiallayers. As such, for example, one layer disposed over or under anotherlayer may be directly in contact with the other layer or may have one ormore intervening layers. Moreover, one layer disposed between two layersmay be directly in contact with the two layers or may have one or moreintervening layers. In contrast, a first layer “on” a second layer is incontact with that second layer. Additionally, the relative position ofone layer with respect to other layers is provided assuming operationsare performed relative to a substrate without consideration of theabsolute orientation of the substrate.

For example, the light absorption region 110 in FIG. 1B may be describedherein as positioned “between” the incident light as shown and themirror 130, even though not all portions of the light absorption region110 (e.g., the light absorption layer 114) are directly located on thesame planar surface (e.g., on top of a substrate) as the incident lightand the mirror 130. For another example, the light absorption layer 114in FIG. 1B may be described herein as disposed “over” the cavity layer112, even though the light absorption layer 114 may not directly contactthe cavity layer 112, such as the case where an interfacial layer (notshown in FIG. 1B) is sandwiched between the light absorption layer 114and the cavity layer 112.

Also, except where otherwise indicated or made apparent from thecontext, some of the terms may be used herein interchangeably. Forexample, the term “attenuation coefficient” or “absorption coefficient”of a light absorption region generally means the coefficient or constantrepresenting, when light travels in the light absorption region, howmuch amount of light energy remains (in percentages as compared to thelight's initial incident energy) after the light being attenuated insidethe light absorption region. The term “round-trip” or “one-circulation”means that the light travels from a first end (e.g., of a lightabsorption region) to a second end, and then returns (e.g., as beingreflected) from the second end back to the first end.

The term “reflect” means that the reflectivity for an incident lightbeam in at least one specific wavelength range, for example, in the nearinfrared region, is greater than 0%.

FIG. 2 illustrates a cross-sectional view showing relevant details ofimplementing a photonic lock based photodetector 200 in accordance withsome embodiments. The photodetector 200 includes a light absorptionregion 210, a first mirror (or a reflective region) 220, and a secondmirror 230. For purposes of explanation, the following discussionassumes that the photodetector 200 is a butt-coupling typephotodetector; however, the technique introduced here can be applied toany suitable semiconductor photodetector.

As mentioned above, for a conventional photodetector, its lightabsorption region needs to be long enough within the incident light'stravel direction in order to absorb enough light. This limitation placesa bottleneck on the bandwidth such conventional photodetector can reach.Accordingly, the photodetector 200 includes a photonic lock mechanism toconfine the light inside the light absorption region 210 so that thelight stays in the light absorption region 210 until the light iscompletely or nearly completely absorbed by the light absorptionmaterial. To this end, one aspect of the technique is to position twomirrors (e.g., the first mirror 220 and the second mirror 230), onemirror on each side of the light absorption region 210, so as to form aphotonic lock. The photonic lock confines the incident light inside thelight absorption region 210, and therefore the incident light can travelmultiple times (by being reflected between the mirrors 220 and 230)inside the light absorption region 210 for almost all the light to beabsorbed

More specifically, in order to “lock” the light inside the lightabsorption region 210, the photonic lock is configured such that anamount of energy of light escaped from the first mirror 220, after beingreflected back by the second mirror 230, is to be zero or near zero. Inorder to reach such lock condition, the optical parameters of the firstmirror 220, together with that of the second mirror 230 and the lightabsorption region 210, are configured such that a substantial amount ofenergy of the incident light is captured by or confined within the lightabsorption region 210. In one or more embodiments, (a) a reflectivity ofthe first mirror 220, (b) a reflectivity of the second mirror 230, and(c) an attenuation coefficient of the light absorption region 210 areconfigured together such that, when the incident light enters the lightabsorption region 210 and reflects between the first mirror 220 and thesecond mirror 230, the light resonates and becomes substantially lockedin the light absorption region 210.

With reference to FIG. 2, some physical principles operating behind thephotonic lock condition for implementing the photonic lockphotodetectors are now described. As shown in FIG. 2, it is assumed thatthe incident light enters into the light absorption region 210 throughthe first mirror 220. Note that, in one or more embodiments, the firstmirror 220 is a partial reflector that allows a portion of the light(and its energy) transmitted into the cavity, and the second mirror 230is a total reflector that reflects a majority of the light (and itsenergy) back into the light absorption region 210. As such, in one ormore embodiments of the photodetector 200, the first mirror 220'sreflectivity is smaller than the second mirror 230's reflectivity. Inthe following discussion, it is assumed that the second mirror 230 is aperfect reflector (i.e., its reflectivity reaches 100%) for simplicity.If the second mirror 230's reflectivity is less than perfect, thetechnique introduced here can still be effective; however, theefficiency may degrade due to the second mirror 230's imperfection. Alsonote that, although not depicted in FIG. 2, the light absorption region210 may include one or more materials and/or structures. For example, incertain embodiments, the light absorption region 210 can employ aphotodiode structure or an avalanche photodiode structure.

Now, assuming that the transmissivity of the first mirror 220 is T, andthat the reflectivity of the first mirror 220 is R1, then a scatteringmatrix (or an “S-matrix”) can be used representing the steady state ofthe above-said structure when the incident light enters:

$\begin{matrix}{\begin{pmatrix}\sqrt{B\; 1} \\\sqrt{B\; 2}\end{pmatrix} = {\begin{pmatrix}{- \sqrt{R\; 1}} & \sqrt{T} \\\sqrt{T} & \sqrt{R\; 1}\end{pmatrix}\begin{pmatrix}\sqrt{A\; 1} \\\sqrt{A\; 2}\end{pmatrix}}} & {{Eq}.\mspace{14mu}(1)}\end{matrix}$where A1, A2, B1, and B2 each represent the optical intensity (or power)of the incident light at a different stage, as shown in FIG. 2.

Furthermore, because the law of conservation of energy, R1 and T has thefollowing relationship:R1+T=1  Eq. (2)

Now, let α represent a one-circulation attenuation coefficient of thelight absorption region 210. As used herein, the one-circulationattenuation coefficient of the light absorption region 210 representshow much light remains in the light absorption region 210 when the lighttravels from entering the light absorption region 210 to exiting thelight absorption region 210, having been reflected by the second mirror230, assuming the exiting point of the incident light is the same as theentry point.

Furthermore, let A2 represent the optical intensity of the incidentlight after being passed into the light absorption region 210, throughthe light absorption region 210, to the second mirror 230, and beingreflected from the second mirror 230, through the light absorptionregion 210 again, to immediately before exiting the light absorptionregion 210. Then, A2 can be denoted by:A2=B2α  Eq. (3)In other words, when the light enters the light absorption region 210, αindicates the ratio of remainder light energy A2 over entrance lightenergy B2 in one circulation.

Note that, in some examples, a can be expressed as a function of R2, thereflectivity of the second mirror 230:α≈R2e ^(−n) ^(i) ^(k) ⁰ ^(4d)  Eq. (4)where n_(i) is the imaginary part of cavity refractive index, k₀ is thefree-space wavenumber, and d is the cavity length (i.e., the distancebetween the first mirror 220 and the second mirror 230).

According to the technique introduced here, a substantial amount ofenergy of the incident light is to be captured by or confined within thelight absorption region 210 so as to reach the aforementioned photoniclock condition. Assuming no phase change is introduced by the first andsecond mirrors for purposes of discussion herein, when the lightresonates in the light absorption region 210, the one-circulation phasechange θ is equal to 2 mπ (where m is integer), and therefore, bycombining equation (2) and equation (3) into the S-matrix in equation(1), the following equation results:B1/A1=(√{square root over (α)}−√{square root over (R1)})²/(1−√{squareroot over (α)}√{square root over (R1)})²  Eq. (5)

It can be seen from equation (5) that, if R1=α, then B1=0. Consequently,to reach a photonic lock condition as described here, an amount ofenergy of light escaped from the first reflective region (e.g., B1),after the light being reflected by the second reflective region, shouldbe substantially zero, meaning that a substantial amount of energy ofthe incident light is captured by or confined within the lightabsorption region 210. Accordingly, one or more embodiments of the firstmirror 220 is configured to exhibit a reflectivity R1 that substantiallyequals a one-circulation attenuation coefficient of the light absorptionregion α.

With the above steady-state description in mind, the following describesa transient-state example to further illustrate the physical principlesbehind the photonic lock photodetectors. Assuming R1 is 10%, if theone-circulation attenuation coefficient α is designed to be 10% as well,then photodetector 200 can reach the aforementioned photonic lockcondition “α=R1” when it operates. Consider the following scenario,after an incident light passes through the first mirror 220 and becomes90%, it travels through the light absorption region 210 while beingabsorbed by the region 210, gets reflected from the second mirror 230,and makes another pass through the region 210. Since a is designed to be10%, before this light exits the region 210 and meets the first mirror220 again, the one-circulation optical intensity of the light becomes90%×10%=9%. Because the first mirror 220 is a reciprocal structure, whenthis light encounters the first mirror 220, 0.9% (which is 9%×10%) ofthe light may appear to be reflected back into the region 210 and 8.1%of the light may appear to pass through the first mirror 220 withoutbeing absorbed by the cavity.

However, despite the amount of energy the light appears to leak orescape from the first mirror 220 being 10% (e.g., on “zero pass”) and8.1% (e.g., on “first pass”), it is observed here that these lightsescape at different phases. More specifically, these escaped lightscreate destructive interferences to each other, and therefore the energythat actually leaks outside the light absorption region 210 can besmaller than what it appears in the above calculations (e.g., the sum of10% and 8.1%).

According to the technique disclosed here, when the optical parametersof the first mirror 220, the light absorption region 210, and the secondmirror 230 are configured together such that the photodetector 200 canreach the photonic lock condition (i.e., “α=R1”) when it operates, afternumerous light passes, essentially all the light that leaks out cancelseach other because of destructive interference, meaning that almost allthe power of the original incident light A1 is trapped inside andabsorbed by the light absorption region 210.

Note that, although many non-ideal factors such as process condition andvariation may exist, the technique introduced here can still bringsimilar benefits even when there is a slight deviation from the ideal“α=R1” case, only that the confinement efficiency may be lower.

FIG. 3 illustrates a flow diagram showing an example of a design process300 that may be implemented by an electronic design automation (EDA)software application for designing a photonic lock based photodetector(e.g., the photodetector 200). Note that, while methods introduced hereinclude a number of steps or operations that are described and/ordepicted as performed in a specific order, these methods may includemore or fewer steps, which may be performed in serial or in parallel.Also, an order of two or more steps may be changed, performance of twoor more steps may overlap, and two or more steps may be combined into asingle step.

First, a dimension and a material for a light absorption region can bedetermined (310) based on a plurality of design factors. The designfactors include a target light's wavelength, its spot size, andcharacteristics of the waveguide or coupling device (e.g., the opticalfiber on top of a grating, or the waveguide coupled to the firstmirror).

Then, a reflector design for the second mirror can be selected (320).The second mirror is to reflect a majority of the light back into thelight absorption region. In one or more embodiments, the second mirroris designed to have as high reflectivity as allowable (i.e., as close to100% as possible) by the fabrication process. Some examples of thesecond mirror can include a DBR mirror, a loop-mirror, a corner mirror,a metal mirror, an oxide mirror, a nitride mirror, a tapered DBRstructure, or a suitable combination of the above. According to certainembodiments, the second mirror's reflectivity is higher than 50%.

Optionally, in those embodiments that have grating over the lightabsorption region for example, a standing wave pattern inside the lightabsorption cavity can be calculated using known methods so that thegrating design (e.g., its reflectivity index) can be selected oradjusted based on the calculation. Other types of known grating designcan also be incorporated over the light absorption cavity.

Next, a one-circulation attenuation coefficient (α) of the lightabsorption region can be determined (330) based on the results of steps310 and 320. The one-circulation attenuation coefficient represents howmuch light (in terms of its optical intensity) remains in the lightabsorption region when the light travels from entering the lightabsorption region to exiting the light absorption region, having beenreflected by the second reflective region and assuming the exiting pointis on the same end of the light absorption region as the entry point.

Thereafter, a reflector design for a first mirror can be selected (340).As discussed above, a reflectivity of the first mirror is configured tobe substantially equal to the one-circulation attenuation coefficient(α) of the light absorption region. Similar to the second mirror,designs the first mirror can include a DBR mirror, a loop-mirror, acorner mirror, a metal mirror, an oxide mirror, a nitride mirror, atapered DBR structure, or a suitable combination of the above. Accordingto certain embodiments, the first mirror's reflectivity is lower than50% or lower than the reflectivity of the second mirror. It is notedthat, because the light can be trapped inside the light absorptioncavity, the length or the thickness of the light absorption cavityitself can become relatively short as compared to traditionalphotodetectors. In one example, the length of the light absorptionregion is shorter than 1 μm. In other examples, the length can bereduced to hundred-nanometer range and still enjoy high responsivity andbandwidth.

Although not depicted in FIG. 3, a designer (or the EDA softwareapplication) may run simulations to verify and, if necessary, repeat theabove steps to adjust or optimize various design parameters in order tobetter suit the application's need.

Also, note that the exact dimensions of various components in a photoniclock based photodetector (e.g., the length, width, and thicknessthereof) should be so adapted that they match or at least as be as closeas possible to the aforementioned photonic lock condition. Suchcondition depends on various factors including the spot size of theincident light, the mode of the light, the wavelength of the light, thematerial of the absorption cavity region, the material and reflectivityof the mirrors, and so forth. These choices may affect the confinementefficiency (henceforth the responsivity and bandwidth); however, thesedesign choices are a part of the optimization process, which does notchange the aforementioned operating principles of the photonic lock.Therefore, a person skilled in this art will be able to apply thetechnique introduced here in performing design optimization.

FIG. 4 illustrates a cross-sectional view of an example implementationof a butt-coupling photodetector 400 incorporating the techniqueintroduced here. The photodetector 400 includes a substrate 407, awaveguide 405, a first mirror region (or reflective region) 420, a lightabsorption region 410, and a second mirror region 430. With reference tothe components shown in FIG. 4, a method for forming the photodetector400 is introduced.

Firstly provided is the substrate 407, which can be silicon dioxide(SiO₂) or other suitable oxide. In variations, the substrate 407 maycomprise other materials, for example, gallium arsenide (GaAs), glass,polymer, or oxynitride.

As shown in FIG. 4, the waveguide 405 is formed over the substrate 407.The waveguide 405 is configured to receive an incident light (e.g.,which may be an optical signal from an optical fiber, not shown in FIG.4) and to pass the incident light through the first reflective region420 into the light absorption region 410. In one embodiment, thewaveguide 405's material comprises silicon (Si). Additionally oralternatively, other suitable materials can be used, for example,aluminum gallium arsenide (AlGaAs), nitride, or polymer. In someembodiments, the waveguide 405 is a single-mode silicon waveguide. Notethat, in general, the waveguide material needs to exhibit a smallerabsorption rate than that of the absorption layer itself at the intendedoperational wavelength. In some examples, the material of the waveguidelayer has a larger band gap as compared to that of the absorption layer.

The first reflective region 420 and the second reflective region 430 canbe formed over the substrate 407. As shown, the first reflective region420 and the second reflective region 430 can be formed on the sameplanar surface. In some examples where the first and second reflectiveregions 420 and 430 are made of the same material as the waveguide 405,all three structures can be formed (e.g., on or over the substrate 407)using the same step or steps.

Next, the light absorption region 410 is formed between the first andsecond reflective regions 420 and 430. For example, a light absorptionmaterial, such as germanium, silicon-germanium, or other Group III-Vmaterials, can be disposed or chemically deposited on substantially thesame planar surface as the first and second reflective regions 420 and430 (e.g., over the substrate 407). The light absorption region 410 ispositioned such that it can absorb light that passes through the firstreflective region 420 and reflects between the first reflective region420 and the second reflective region 430. In the butt-coupling exampleof FIG. 4, the light absorption material is formed on substantially thesame planar surface as the first and second reflective regions 420 and430. This can be implemented by one or more known etching and chemicaldeposition processes. Note that the light absorption region 410 can alsobe formed prior to the formation of the first and second reflectiveregions 420 and 430.

As mentioned above, the first reflective region 420 is configured toexhibit a reflectivity that causes an amount of energy of light escapedfrom the first reflective region, after being reflected by the secondreflective region, to be substantially zero. In one or more embodiments,the first reflective region 420's reflectivity is configured to besubstantially equal to a one-circulation attenuation coefficient of thelight absorption region 410.

FIG. 5 illustrates a cross-sectional view of an example implementationof an evanescent-coupling photodetector 500 incorporating the techniqueintroduced. Similar to the photodetector 400, the photodetector 500includes a substrate 507, a waveguide 505, a first reflective region520, a light absorption region 510, and a second reflective region 530.The structures and manufacturing processes for the waveguide, thesubstrate, and the first and second reflective regions of thephotodetector 500 are similar to those of the photodetector 400introduced above.

For the light absorption region 510, one difference between theevanescent-coupling photodetector 500 and the butt-couplingphotodetector 400 is that a cavity layer 512 (i.e., instead of the lightabsorption material) is first disposed on substantially the same planarsurface as the first and second reflective regions 520 and 530. This canbe formed during the step of forming the first and the second reflectiveregions 520 and 530. In some embodiments, the cavity layer 512 includesthe same material as the first and second reflective regions 520 and530.

Then, optionally, one or more interfacial layers (e.g., ananti-reflection coating, material stacks (for better crystal sizematching), and/or a quarter-wavelength layer) or grating layers 513 canbe disposed over the cavity layer 512 to improve optical characteristicsof the interface between the cavity layer 512 and a light absorptionlayer 514. In some examples, the grating layer 513 can be designed tomatch the standing wave pattern inside the cavity layer 512. By matchingthe wave pattern, the grating layer 513 may function similarly to anantenna, providing an efficient conduit for the light to leave thecavity layer 512 into the light absorption layer 514. More specifically,in some embodiments, the grating layer 513 can be designed such that aperiod of the grating equals two times the distance between two maximumpower points of the standing wave pattern of the light traveling in thecavity layer 512. In this way, all point wave-fronts emitted fromindividual grating structure may be combined into a planar wave front,which in this turn propagates upward into the light absorption layer514.

Thereafter, the light absorption layer 514 which contains the lightabsorption material can be disposed over the cavity layer 512. It isnoted that this process of fabricating the photodetector 500 may beadvantageous over the process of fabricating the photodetector 400 incertain implementations where the material of the light absorption layer514 can be more easily disposed over the cavity layer 512 (whether it iswith or without the interfacial layer 513) than over the substrate 507.

Although not depicted in FIG. 5, in another embodiment, the incidentlight can enter from the top of the absorption layer 514. In accordancewith the technique introduced here, the amount of light not absorbed onthe first pass can still become confined (or “locked”) in the cavitylayer 512 while being evanescently absorbed by the absorption layer 514.

FIG. 6 illustrates a cross-sectional view of an example implementationof a normal-incidence photodetector 600 incorporating the techniqueintroduced here. Similar to the photodetector 400, the photodetector 600includes a first reflective region 620, a light absorption region 610,and a second reflective region 630. The structures for the first andsecond reflective regions of the photodetector 600 are similar to thoseof the photodetector 400 introduced above. With reference to thecomponents shown in FIG. 6, a method for forming the photodetector 600is introduced.

Firstly provided is a substrate, which can be a silicon dioxide (SiO₂)or other suitable oxide. In variations, the substrate may comprise othermaterials, for example, gallium arsenide (GaAs). Note that, in certainembodiments, after the reflective and absorption structures are formed,the substrate can be removed by an etching process, hence not shown inFIG. 6.

Then, the second reflective region 630 is formed over the substrate. Asmentioned above, some examples of the second reflective region 630 caninclude a DBR mirror, a metal mirror, an oxide mirror, a nitride mirror,a tapered DBR structure, or a suitable combination of the above.According to certain embodiments, the second reflective region 630'sreflectivity is higher than 50%. In one or more embodiments, the secondreflective region 630 is designed to have as high reflectivity asallowable (i.e., as close to 100% as possible) by the fabricationprocess.

Next, the light absorption region 610 is formed over the secondreflective region 630. For example, a light absorption material, such asgermanium, silicon-germanium, or other Group III-V materials, can bedisposed, bonded, attached, or otherwise chemically deposited over thesecond reflective region 630.

Thereafter, the first reflective region 620 is formed over the lightabsorption region 610. As mentioned above, the first reflective region620 is configured to exhibit a reflectivity that causes an amount ofenergy of light escaped from the first reflective region 620, afterbeing reflected by the second reflective region 630, to be substantiallyzero. In one or more embodiments, the first reflective region 620'sreflectivity is configured to be substantially equal to aone-circulation attenuation coefficient of the light absorption region610.

Note that, the above example process is merely one example of formingthe normal-incidence photodetector 600. As a variation, the secondreflective region 630 can be formed by first removing the portion ofsubstrate beneath the light absorption structure 610, and then coatingthe etched opening with a reflective material (e.g., metal) to functionas the second reflective region 630, and this fabrication process can beperformed after the first reflective region is made.

In one variation, the first reflective region 620 can be formed on thesubstrate, and then the light absorption region 610 is disposed on topof the first reflective region 620, which can be followed by disposingthe second reflective region 630 on top of the light absorption region610. Thereafter, the substrate can be removed, thereby forming thestructure 600 as shown in FIG. 6.

In another variation, the first reflective region 620 can be formed byfirst removing the substrate portion beneath the light absorption region610, and then coating the etched opening with one or more reflectivematerials (e.g., multiple layers of oxide and nitride to form a DBRmirror structure) so as to function as the first reflective region 620.In some embodiments, this removing and coating fabrication processes canbe performed after the second reflective region 630 is fabricated.

Subsequently, an optical coupling apparatus (not shown in FIG. 6) can becoupled to the first reflective region 620. It is configured to receivean incident light (e.g., which may be an optical signal from an opticalfiber, not shown in FIG. 6) and to pass the incident light through thefirst reflective region 620 into the light absorption region 610.

FIGS. 7A-7H illustrate various examples of mirror designs, which can beadapted in the photonic lock based photodetectors introduced here. Forsimplicity, only examples of butt-coupling photodetectors are shown inFIGS. 7A-7H; however, similar techniques can be applied to differenttypes of photodetectors. In addition, although the first and the secondmirrors in each of FIGS. 7A-7H are illustrated as embodying a certaincombination of mirror designs, all the mirror designs illustrated here,as well as other well-known mirror designs, can be adapted forimplementing any mirror region (or reflective region) described in thepresent disclosure.

FIG. 7A illustrates a cross-sectional view of an example implementationof a butt-coupling photodetector 700A. The first mirror 720 can be ahigh reflection tapered DBR or a low reflection trench. It is noted thatusing a tapered DBR structure as the second mirror 730 may provide abetter light locking mechanism as compared to other types of mirror.

FIG. 7B illustrates a cross-sectional view of an example implementationof a butt-coupling photodetector 700B. The second mirror 731 isimplemented via a coating of metal, and the first mirror 721 isimplemented using a tapered DBR structure.

FIG. 7C illustrates a cross-sectional view of an example implementationof a butt-coupling photodetector 700C. The second mirror 732 includes adielectric layer, for example, a quarter wavelength oxide layer, and thefirst mirror 722 is implemented using a trench structure.

FIG. 7D illustrates a cross-sectional view of an example implementationof a butt-coupling photodetector 700D. The second mirror 733 includes atrench, and the first mirror 723 is implemented using a tapered DBRstructure.

FIG. 7E illustrates a perspective view of an example implementation of abutt-coupling photodetector 700E. The second mirror 734 includes acorner mirror that is made of the same material as the light absorptionregion 714. As shown in FIG. 7E, the first mirror 724 is implementedusing a trench structure.

FIG. 7F illustrates a perspective view of an example implementation of abutt-coupling photodetector 700F. The second mirror 735 includes acorner mirror that is made of a different material than the lightabsorption region 715. As shown in FIG. 7F, the first mirror 725 isimplemented using a tapered DBR structure.

FIG. 7G illustrates a perspective view of an example implementation of abutt-coupling photodetector 700G. As shown in FIG. 7G, the structure ofthe photodetector 700G is designed such that the interface between thewaveguide 705 and the light absorption region 716 essentially doubles asthe first mirror 726; that is to say, the same component functions bothas the waveguide 705 as well as the first mirror 726.

FIG. 7H illustrates a perspective view of an example implementation of abutt-coupling photodetector 700H. As shown in FIG. 7H, the structure ofthe photodetector 700H is designed such that an end of the lightabsorption region 717 that is distal from the first mirror 727essentially functions as the second mirror 737; that is to say, thereflectivity resulting from the difference of materials between thelight absorption region 717 and whatever material (which can also bevoid, such as depicted in FIG. 7H) being disposed at the distal end ofthe light absorption region 717 (i.e., where the second mirror 737 isdepicted) can make the distal end of the light absorption material 717function as the second mirror 737. As shown in FIG. 7H, the first mirror727 is implemented using a tapered DBR structure.

In the above described manner, the technique and apparatus introducedhere provide a photonic lock mechanism to confine the incident lightinside the light absorption region of the photodetector, therebyachieving both high responsivity and high bandwidth. The techniqueintroduced here also helps shrink the overall size of thephotodetectors.

FIG. 8 is a high-level block diagram showing an example of a computingdevice 800 that can implement at least some portion of the designprocess 300 of FIG. 3. Additionally or alternatively, the computingdevice 800 can represent an environment within which an embodiment ofthe photonic lock based photodetectors introduced here can beimplemented.

In the illustrated embodiment, the computing system 800 includes one ormore processors 810, memory 811, a communication device 812, and one ormore input/output (I/O) devices 813, all coupled to each other throughan interconnect 814. The interconnect 814 may be or may include one ormore conductive traces, buses, point-to-point connections, controllers,adapters and/or other conventional connection devices, such as opticalcommunication elements that may include a photonic lock basedphotodetector introduced here. The processor(s) 810 may be or mayinclude, for example, one or more general-purpose programmablemicroprocessors, microcontrollers, application specific integratedcircuits (ASICs), programmable gate arrays, or the like, or acombination of such devices. The processor(s) 810 control the overalloperation of the computing device 800. Memory 811 may be or may includeone or more physical storage devices, which may be in the form of randomaccess memory (RAM), read-only memory (ROM) (which may be erasable andprogrammable), flash memory, miniature hard disk drive, or othersuitable type of storage device, or a combination of such devices.Memory 811 may store data and instructions that configure theprocessor(s) 810 to execute operations in accordance with the techniquesdescribed above. The communication device 812 may be or may include, forexample, an Ethernet adapter, cable modem, Wi-Fi adapter, cellulartransceiver, Bluetooth transceiver, or the like. The communicationdevice 812 may also include optical communication or optical sensingelements such as a photonic lock based photodetector. Depending on thespecific nature and purpose of the processing device 800, the I/Odevices 813 can include devices such as a display (which may be a touchscreen display), audio speaker, keyboard, mouse or other pointingdevice, microphone, camera, etc. A photonic lock based photodetectorintroduced here may be included as (e.g., optical sensing) components inone or more of the I/O devices 813.

Unless contrary to physical possibility, it is envisioned that (i) themethods/steps described above may be performed in any sequence and/or inany combination, and that (ii) the components of respective embodimentsmay be combined in any manner.

The techniques introduced above can be implemented by programmablecircuitry programmed/configured by software and/or firmware, or entirelyby special-purpose circuitry, or by a combination of such forms. Suchspecial-purpose circuitry (if any) can be in the form of, for example,one or more application-specific integrated circuits (ASICs),programmable logic devices (PLDs), field-programmable gate arrays(FPGAs), etc.

Software or firmware to implement the techniques introduced here may bestored on a machine-readable storage medium and may be executed by oneor more general-purpose or special-purpose programmable microprocessors.A “machine-readable medium”, as the term is used herein, includes anymechanism that can store information in a form accessible by a machine(a machine may be, for example, a computer, network device, cellularphone, personal digital assistant (PDA), manufacturing tool, any devicewith one or more processors, etc.). For example, a machine-accessiblemedium can include recordable/non-recordable media (e.g., read-onlymemory (ROM), random access memory (RAM), magnetic disk storage media,optical storage media, flash memory devices, etc.).

Note that any and all of the embodiments described above can be combinedwith each other, except to the extent that it may be stated otherwiseabove or to the extent that any such embodiments might be mutuallyexclusive in function and/or structure.

Although the present invention has been described with reference tospecific exemplary embodiments, it will be recognized that the inventionis not limited to the embodiments described, but can be practiced withmodification and alteration within the spirit and scope of the appendedclaims. Accordingly, the specification and drawings are to be regardedin an illustrative sense rather than a restrictive sense.

What is claimed is:
 1. A silicon-based germanium photodetector devicecomprising: a first reflective region; a second reflective region; and alight absorption region positioned between the first and secondreflective regions so as to absorb light passed through the firstreflective region and reflected between the first and second reflectiveregions, wherein the first reflective region is such constructed that areflectivity of the first reflective region is substantially equal to aone-circulation attenuation coefficient of the light absorption region,wherein the first reflective region is responsible for causing adestructive interference upon light escaping from the first reflectiveregion so as to reduce an amount of light that escapes from the firstreflective region, wherein the second reflective region includes acombination of a dielectric layer and a metal coating, the dielectriclayer (1) having at least one of: oxide, or nitride, and (2) beingpositioned closer to the light absorption region than the metal coating,wherein the combination causes a reflectivity of the second reflectiveregion to exceed ninety percent (90%), and wherein the light absorptionregion includes germanium.
 2. The device of claim 1, wherein (a) thefirst reflective region's reflectivity, (b) the second reflectiveregion's reflectivity, and (c) an attenuation coefficient of the lightabsorption region are collectively configured such that the lightresonates between the first and second reflective regions.
 3. The deviceof claim 1, wherein the first reflective region causes a total amount oflight energy that escapes from the first reflective region to be lessthan 10% of incident light energy.
 4. The device of claim 3, wherein theone-circulation attenuation coefficient indicates a ratio of remainderlight energy over entrance light energy in one circulation.
 5. Thedevice of claim 1, wherein the first reflective region's reflectivity isless than a reflectivity of the second reflective region.
 6. The deviceof claim 1, further comprising: a waveguide configured to pass the lightthrough the first reflective region into the light absorption region. 7.The device of claim 1, wherein the light absorption region comprises alight absorption material that is disposed on substantially the sameplanar surface as the first and second reflective regions.
 8. The deviceof claim 1, wherein the light absorption region comprises (a) a cavitylayer disposed on substantially the same planar surface as the first andsecond reflective regions, and (b) a light absorption layer that isdisposed over the cavity layer, wherein the light absorption layer andthe cavity layer are of different materials.
 9. The device of claim 1,wherein the first reflective region is disposed over the lightabsorption region, and wherein the light absorption region is disposedover the second reflective region.
 10. The device of claim 1, whereinthe light absorption region comprises a photodiode or an avalanchephotodiode layered structure.
 11. The device of claim 1, wherein thefirst and second reflective regions are selected from a group including:a distributed Bragg reflector (DBR), a metallic reflector, a cornermirror, and a reflection trench.
 12. The device of claim 1, wherein thelight absorption region comprises a Group III-V compound, germanium, ora combination thereof.
 13. The device of claim 1, wherein the firstreflective region is responsible to reduce a back reflection of incidentlight from the first reflective region when the incident light firstenters the first reflective region.
 14. The device of claim 13, whereinthe back reflection is reduced to substantially zero.
 15. A method forforming a photodetector device, the method comprising: forming a lightabsorption region, a first reflective region and a second reflectiveregion over a substrate, wherein the first and second reflective regionsare formed on substantially the same planar surface; and forming a lightabsorption region between the first and second reflective regions so asto absorb light passed through the first reflective region and reflectedbetween the first and second reflective regions, wherein the firstreflective region is such constructed that a reflectivity of the firstreflective region is substantially equal to a one-circulationattenuation coefficient of the light absorption region, wherein thefirst reflective region is responsible for causing a destructiveinterference upon light escaping from the first reflective region so asto reduce an amount of light that escapes from the first reflectiveregion, wherein the second reflective region includes a combination of adielectric layer and a metal coating, the dielectric layer (1) having atleast one of: oxide, or nitride, and (2) being positioned closer to thelight absorption region than the metal coating, wherein the combinationcauses a reflectivity of the second reflective region to exceed ninetypercent (90%), and wherein the light absorption region includesgermanium.
 16. The method of claim 15, wherein (a) the first reflectiveregion's reflectivity, (b) the second reflective region's reflectivity,and (c) an attenuation coefficient of the light absorption region arecollectively configured such that the light resonates between the firstand second reflective regions.
 17. The method of claim 15, wherein thefirst reflective region causes a total amount of light energy thatescapes from the first reflective region to be less than 10% of incidentlight energy.
 18. The method of claim 15, wherein the first reflectiveregion's reflectivity is less than a reflectivity of the secondreflective region.
 19. The method of claim 15, further comprising:forming a waveguide over the substrate, wherein the waveguide isconfigured to pass the light through the first reflective region intothe light absorption region.
 20. The method of claim 15, wherein formingthe light absorption region comprises: disposing a light absorptionmaterial on substantially the same planar surface as the first andsecond reflective regions.
 21. The method of claim 15, wherein formingthe light absorption region comprises: during the step of forming thefirst and the second reflective regions, disposing a cavity layer onsubstantially the same planar surface as the first and second reflectiveregions; and disposing a light absorption layer over the cavity layer,wherein the light absorption layer and the cavity layer are of differentmaterials.
 22. A method for forming a silicon-based germaniumphotodetector device, the method comprising: forming a light absorptionregion; and forming a first and a second reflective regions; wherein thelight absorption region is positioned between the first and secondreflective regions so as to absorb light passed through the firstreflective region and reflected between the first and second reflectiveregions, and wherein the first reflective region is such constructedthat a reflectivity of the first reflective region is substantiallyequal to a one-circulation attenuation coefficient of the lightabsorption region, wherein the first reflective region is responsiblefor causing a destructive interference upon light escaping from thefirst reflective region so as to reduce an amount of light that escapesfrom the first reflective region, wherein the second reflective regionincludes a combination of a dielectric layer and a metal coating, thedielectric layer (1) having at least one of: oxide, or nitride, and (2)being positioned closer to the light absorption region than the metalcoating, wherein the combination causes a reflectivity of the secondreflective region to exceed ninety percent (90%), and wherein the lightabsorption region includes germanium.
 23. The method of claim 22,wherein (a) the first reflective region's reflectivity, (b) the secondreflective region's reflectivity, and (c) an attenuation coefficient ofthe light absorption region are collectively configured such that thelight resonates between the first and second reflective regions.
 24. Themethod of claim 22, wherein the first reflective region causes a totalamount of light energy that escapes from the first reflective region tobe less than 10% of incident light energy.
 25. The method of claim 22,wherein the first reflective region's reflectivity is less than areflectivity of the second reflective region.
 26. An optoelectronicsystem for sensing optical signals, the system comprising: amicrocontroller; a memory; and an optoelectronic apparatus including asilicon-based germanium photodetector that includes: a first reflectiveregion; a second reflective region; and a light absorption regionpositioned between the first and second reflective regions so as toabsorb light passed through the first reflective region and reflectedbetween the first and second reflective regions, wherein the firstreflective region is such constructed that a reflectivity of the firstreflective region is substantially equal to a one-circulationattenuation coefficient of the light absorption region, wherein thefirst reflective region is responsible for causing a destructiveinterference upon light escaping from the first reflective region so asto reduce an amount of light that escapes from the first reflectiveregion, wherein the second reflective region includes a combination of adielectric layer and a metal coating, the dielectric layer (1) having atleast one of: oxide, or nitride, and (2) being positioned closer to thelight absorption region than the metal coating, wherein the combinationcauses a reflectivity of the second reflective region to exceed ninetypercent (90%), wherein the light absorption region includes germanium,wherein (a) the first reflective region's reflectivity, (b) the secondreflective region's reflectivity, and (c) an attenuation coefficient ofthe light absorption region are collectively configured such that thelight resonates between the first and second reflective regions, andwherein the one-circulation attenuation coefficient indicates a ratio ofremainder light energy over entrance light energy in one circulation.